Disable Snappy support by default

[FrancescAlted]

Snappy is a C++ codec that creates unnecessary complexity in building tools (see Blosc/python-blosc#127 (comment)), so provided that there are better codecs giving better speed and compression (e.g. LZ4), I propose it to be disabled by default.

[nmoinvaz]

This looks like it is done, do you want to go a step further and remove building snappy from sources but allow testing against external snappy (similar to c-blosc2)?

[FrancescAlted]

Yes, that would be cool.